Method for surface treatment of a protectively coated substrate

ABSTRACT

A method is shown for surface-treating a protectively coated substrate ( 1 ), more particularly steel strip, for the purpose of enhancing the strength of adhesion with an adhesive ( 10 ), where the at least partly multi-phase protective coat ( 2 ) comprises at least zinc and magnesium. In order to provide a particular suitability for adhesive bonding, it is proposed that the protective coat ( 2 ) be subjected to a pickling treatment, directed substantially at at least one intermetallic Zn—Mg phase ( 4 ) of the protective coat ( 2 ), with acidic pickle, more particularly acid.

TECHNICAL FIELD

The invention relates to a method for surface treatment of a protectively coated substrate, particularly steel strip, for improvement of the adhesion strength, with an adhesive, wherein the at least partly multi-phase protective coating comprises at least zinc and magnesium.

STATE OF THE ART

To improve the adhesive suitability of a steel sheet that is coated with a zinc/magnesium layer for corrosion protection, a method is known from the state of the art (DE102008004728A1), in which a phosphatization solution is applied to the zinc/magnesium layer. In this way, a fine crystal structure for binding of the adhesive is supposed to be created on the protective coating. It is disadvantageous, in such a method, that during phosphatization, the formation of zinc (magnesium) phosphate crystals cannot be excluded, and this increases the risk of tearing of the phosphate layer under force stress. Therefore no stable glued connection can be guaranteed in this way. Furthermore, it is known from JP9241828A to carry out surface treatment of a sheet metal protectively coated with Zn—Mg, using phosphoric acid. This treatment is selectively directed at the magnesium component of the protective coating, in order to avoid negative effects of magnesium with regard to varnishing. This surface treatment cannot contribute to improvement of the adhesion suitability. DE102004045297A1 speaks about treatment of a metallic surface, which can also be coated with Zn and/or Mg. For this purpose, among other things, methane sulfonic acid having a pK_(s) of −0.6 is proposed. However, DE102004045297A1 concerns itself neither with a selection direction of the surface treatment on components of the protective coating, nor with a surface treatment for improvement of adhesion suitability. From JP2007002288A, a treatment of a Zn—Al—Mg protective coating with acid is known, in order to thereby prepare this coating as a varnish base. Selective reduction of the intermetallic phases of this protective coating, in order to thereby improve the adhesion strength, using an adhesive, is not evident from JP207002288A.

PRESENTATION OF THE INVENTION

The invention has therefore set itself the task, proceeding from the state of the art as described, of creating a method with which not only can a surface of a substrate protectively coated at least with zinc and magnesium be prepared for an adhesive connection in fast and cost-advantageous manner, but also, an improvement in adhesion suitability can be made possible in this way.

The invention accomplishes the stated task in that the protective coating is subjected to a pickling treatment that is essentially directed at at least one intermetallic Zn—Mg phase of the protective coating, using an acidic pickle.

If the protective coating is subjected to a pickling treatment that is essentially directed at at least one intermetallic Zn—Mg phase of the protective coating, using an acidic pickle, then the surface of a multi-phase protective coating comprising zinc and magnesium can be prepared in such a manner that in this way, particular adhesion strength with an adhesive comes about. Specifically, according to the invention, not only can a greater surface area for adhesive binding be created in this way, but also, the reaction surface of the protective coating can be improved. This is because metal hydroxides (for example Zn hydroxides) that increasingly occur on the pickled locations of the protective coating are available to the adhesive for formation of the connection. By means of this selective pickling of the finely distributed intermetallic Zn—Mg phases (e.g. MgZn₂, Mg₂Zn₁₁, in the binary and/or ternary eutectic), not only can a simple method for preparation of a protectively coated substrate for an adhesive connection be created, but rather, adhesion suitability can also be clearly improved. The method according to the invention can therefore particularly be characterized in that the protectively coated substrate is subjected to such pickling treatment, that at most parts (particularly the intermetallic Zn—Mg phase) are removed at least from the surface of the multi-phase protective coating, and that therefore a surface structure and reaction surface of the protective coating form, which can ensure better binding of an adhesive to the protective coating or, subsequently, to the substrate. In contrast to the state of the art, a stable connection with an adhesive can be ensured even in the case of steel strip coated at least with zinc and magnesium. In particular, adhesion ruptures known in the case of protective coatings comprising zinc and magnesium can be excluded in this way, so that only cohesion ruptures have to be accepted. According to the invention, a substrate treated in this way can therefore be used in automobile construction, even for glued connections.

Selective pickling treatment can be made possible, for example, by making use of the different physical, mechanical and/or chemical properties of the intermetallic Zn—Mg phase as compared with the other elements of the protective coating—such as, for example, Zn dendrites. Thus, these coatings possibly demonstrate different hardness, brittleness, etc., and the acidic pickle and/or the treatment duration with this acidic pickle can be adjusted to these differences, in order to thereby have a pickling effect essentially on this intermetallic Zn—Mg phase—in other words more as compared with the other elements of the protective coating.

The method can be further refined in that an oxide layer of the protective coating is also removed by pickling during the pickling step for selective pickling of the protective coating. In one method step, according to the invention, the protectively coated substrate can be prepared for its selective pickling, on the one hand, and also, the selective pickling can be carried out. All that is necessary is that an acidic pickle as well as a related pickling duration of this pickling step must be selected in such a manner that these conditions can be met. Furthermore, the method costs can be reduce by means of the use of only one pickling agent.

In that the protective coating is subjected to a pickling treatment essentially directed at MgZn₂, targeted, selective pickling of the protective coating can be made possible, particularly making use of the brittleness of this Laves phase, without thereby having to fear significant impairment of other microstructure parts. The method according to the invention can therefore be characterized, in addition to the improvement in the adhesion suitability of the substrate, also by its comparatively low removal of material. Pickling particularly directed at the eutectic has proven to be particularly advantageous for the creation of a nano-topography at the surface of the substrate. Specifically, it is possible to penetrate into the depths of the protective coating along the dendritic formation of the eutectic that is close to the surface. In this connection, binary eutectic (Zn and MgZn₂) or ternary eutectic (Zn, Al and MgZn₂) of the microstructure of the protective coating can be pickled, whereby the ternary eutectic can be preferred for this, because of its finer structure as compared with the binary eutectic.

It can prove to be advantageous if the protective coating that additionally comprises aluminum is subjected to pickling treatment essentially directed at a eutectic. Specifically, in this way the fine distribution of the particularly ternary eutectic can be utilized to ensure a special surface structure and reaction surface for improved bonding of the adhesive to the protective coating.

Acid as a pickle, with a pK_(s) value less than 0, can particularly distinguish itself for selective pickling of the intermetallic Zn—Mg phase of the protective coating. Furthermore, a continuous method for surface treatment can be ensured with such an acid strength, something that can be particularly utilized for significant reduction of the time required for the method, particularly in the case of protectively coated steel strips. In general, it should also be mentioned that the pKs value is define as the negative decadic logarithm of K_(s), the acid constant.

If a non-oxidizing acid is used as an acid pickle, the method can particularly be characterized by a short pickling time and its pickling attack selectively directed at the intermetallic Zn—Mg phase. In particular, H₂SO₄ as an acid can proven itself as advantageous in this connection. Aside from fast surface treatment, a surface structure can also be created in this way, which is characterized by nano-topography at least in certain regions, and this can allow increased adhesion suitability.

Particular method conditions can occur if the protective coating is subjected to a pickling duration of 0.5 to 20 seconds with an acid having a concentration of 1 to 20 volume-% (or of 1 to 20 volume percent).

Particularly improved method conditions can occur if the protective coating is subjected to a pickling duration of 1 to 15 seconds with an acid having a concentration of 2 to 10 volume-%.

If the product that results from the pickling duration in seconds and the concentration in volume-% lies in the range of 1 to 100, then simple standard operating procedures for the pickling attack on the intermetallic Zn—Mg phase of the protective coating can exist. In particular, the oxide layer of the protective coating can also be removed in this way, in simple manner.

The standard operating procedures for the pickling attack can be further improved if the product that results from the pickling duration in seconds and the concentration in volume-% lies in the range of 6 to 75.

In particular, the invention can be characterized in that an acid having a pK, value of less than 0 is used for surface treatment of an object that is corrosion-protected with a multi-phase protective coating, comprising at least zinc and magnesium, particularly steel strip, for improvement of the adhesion strength, with an adhesive. In contrast to known methods, in which adhesion-imparting agents are applied to the surface or in which phosphatization of the surface is carried out to increase the adhesion suitability of a protectively coated substrate, use of an acid for surface treatment can allow a fast method and a high reactive surface for adhesive binding.

Advantageous method conditions can furthermore occur if the acid is used for a pickling treatment essentially directed at the eutectic of a protective coating that additionally comprises aluminum. In particular, the fine distribution of the ternary eutectic can be distinguished for an improvement of the surface structure and of the reaction surface for a connection with an adhesive.

The invention has furthermore set itself the task of creating an object, particularly a steel strip, having improved adhesion suitability for a stable connection.

The invention accomplishes the stated task with regard to the object in that the protective coating has a microstructure that is reduced by part of the intermetallic Zn—Mg phase, produced by means of selective pickling of the protective coating.

If the protective coating has a microstructure that is reduced by the proportion of intermetallic Zn—Mg phase, not only can a particular surface structure with nano-topography be made possible for adhesive bonding, but also, this surface can additionally be characterized by an improved reaction and adsorption surface for the adhesive. In contrast to the state of the art, a Zn—Mg surface with sufficient binding surface area can thereby be made available to an adhesive—without additional measures—particularly since, as compared with known Zn—Mg surfaces, an increased hydroxide characterization can advantageously be utilized for improved chemical binding. Therefore a stable connection with an adhesive, particularly of a steel strip with an adhesive, cannot be ensured.

Simple design conditions and improved binding properties with an adhesive can occur if the protective coating has a microstructure reduced by the MgZn₂ component.

The surface of the object can be prepared for an adhesive connection in further improved manner if the protective coating, which additionally comprises aluminum, has a microstructure reduced by the proportion of eutectic. In particular, reduction by the ternary eutectic can distinguish itself for this purpose, because of its fine structuring or distribution as compared with the binary eutectic.

BRIEF DESCRIPTION OF THE DRAWING

The object of the invention is shown in the figure, as an example, using an exemplary embodiment. The figures show:

FIG. 1 a transverse microsection of a microstructure of a steel strip coated with zinc-magnesium,

FIG. 2 an enlarged microsection of the steel strip according to FIG. 1, treated according to the method according to the invention, and

FIG. 3 a top view of the steel strip according to FIG. 2.

WAY FOR IMPLEMENTATION OF THE INVENTION

In FIG. 1, a layer structure of a steel strip 1 coated at least with zinc-magnesium for corrosion protection is shown in transverse microsection, as an example. Various methods are known from the state of the art for application of such a protective coating 2, whereby during these methods, a multi-phase coating of zinc 3 and intermetallic Zn—Mg phases 4 forms because of the low miscibility of zinc and magnesium and because of the rapid solidification of the protective coating. This intermetallic Zn—Mg phase 4 can be present in different arrangements and characterizations, namely as an MgZn₂ layer 5, particularly characterized by a clod shape, on the surface of the Zn—Mg coating, or as a eutectic 6, 7, which can be the case, for example, as a binary eutectic 6 (zinc and MgZn₂) or as a ternary eutectic (zinc, aluminum, and MgZn₂). Also, an aluminum phase 8 can exist. The intermetallic Zn—Mg phase 4 reaches all the way to the surface of the protective coating 2, whereby the protective coating 2 forms an oxide layer 9, at least in part. This oxide layer 9 can primarily comprise Mg oxide, Al oxide, and Mg—Al oxide, and can have a smaller proportion of Zn oxide.

Using FIG. 2, the technical effect of the pickling method according to the invention will now be explained in greater detail, as an example: To improve the adhesion strength between the steel strip 1 and an adhesive 10, the protective coating 2 is subjected to a pickling treatment essentially directed at at least one intermetallic Zn—Mg phase 4 (MgZn₂), using an acidic pickle. The intermetallic Zn—Mg phase 4 is therefore reduced to a greater degree, as compared with other parts of the microstructure of the protective coating 2, so that a nano-topography 11 can form by means of this selective pickling of the Zn—Mg phase 4 that is close to the surface, as can be seen in FIG. 3 as a top view of this protective coating 2. The increased roughness of the surface of the protective coating 2 can also be seen in FIG. 3. In particular, an acid having a pK_(s) value less than 0 has proven to be advantageous for such selective pickling attack, because in this way, removal by pickling can take place even in one step, also of the oxide layer 9 (hot oxides and/or cold oxides, which cold oxides can be formed, for example, during re-rolling of the surface before the pickling attack). Thus, it can be seen in FIGS. 2 and 3 that in the region of the reduced microstructure of the protective coating 2, the original oxide layer 9 was also removed. It is advantageous that in these selectively pickled regions of the protective coating 2, metal hydroxides 12, for example Zn hydroxides and Mg hydroxides, also form to an increasing degree, thereby allowing improved chemical binding, in addition to the Van-der-Waals bonds between the protective coating 2 or the steel strip 1 and the adhesive 10.

Because MgZn₂ is present not only as a layer 5 but also in the eutectic 6, 7, it is possible to subject the protective coating to a pickling treatment directed at MgZn₂, for a simple method for the selective reduction of the protective coating 2 and for the formation of Zn and Mg hydroxides 12. In particular, in this way the eutectics 6, 7 can also be reduced; these can form a relief on a nano scale for improved adhesion suitability, on the basis of the fine MgZn₂ distribution.

For this pickling attack on the steel strip 1, H₂SO₄ at a concentration of 2 to 10 volume-% and a pickling duration of 1 to 15 seconds, for example, has proven to be advantageous.

The effect according to the invention is described in greater detail, as an example, using the exemplary embodiments summarized in Table 1: An object or a steel strip 1 is coated using a melt-immersion zinc-plating technology, with a composition comprising zinc, 2 wt.-% (or 2 weight-percent) aluminum, and 2 wt.-% magnesium. A multi-phase protective coating 2 forms on the steel strip 1, as has already been explained in detail in FIG. 1.

TABLE 1 Pickling Concentration Product Acid pK_(s) value time (s) (volume-%) (s * volume-%) HCl −6 3 5 15 HCl −6 5 5 25 HCl −6 10 5 50 HCl −6 15 5 75 HNO3 −1.32 3 5 15 H2SO4 −3 10 1 10 H2SO4 −3 15 1 15 H2SO4 −3 3 2.5 8 H2SO4 −3 5 2.5 13 H2SO4 −3 10 2.5 25 H2SO4 −3 15 2.5 38 H2SO4 −3 3 5 15 H2SO4 −3 5 5 25 H2SO4 −3 10 5 50 H2SO4 −3 15 5 75

First, a previously described steel strip 1 was subjected to a non-oxidizing acid HCl with a pK_(s) value of −6, a concentration of 5 volume-%, and a different pickling duration of 3, 5, 10, and 15 seconds, in each instance. The results demonstrated an excellent surface structure and reaction surface of the protective coating 2 for binding to an adhesive 10. Furthermore, it was possible to also remove the oxide layer 9 of the protective coating 2 in this way. An increase in the pickling duration or a reduced pickling duration was not able to show any significant improvements. The product of pickling duration in seconds and concentration in volume-% (product=pickling duration*concentration) lies in the range between 15 and 75, according to Table 1.

In the next experiment, a steel strip 1 was subjected to an oxidizing acid HNO₃ with a pK_(s) value of −1.23, a concentration of 5 volume-%, and pickling duration of 3 seconds. As compared with the preceding experiment, it was possible to achieve comparably good results, whereby here, a product of pickling duration in seconds and concentration in volume-% of 15 was obtained.

In a final further experiment, such a steel strip 1 was subjected to an acid H₂SO₄ with a pK_(s) value of −3, at different concentrations in volume-%, namely 1, 2.5, and 5 volume-%. The relationship between concentration in volume-% and pickling duration in seconds is clearly evident in Table 1. Starting from a concentration of 2.5 volume-%, it was possible to recognize a clear reduction in the pickling duration from 2 and 5 seconds, respectively. In general, a pickling duration of 3 to 15 seconds was able to demonstrate excellent results, whereby in this experiment, a product of pickling duration in seconds and concentration in volume-% of 8 to 75 was obtained.

According to the invention, it was therefore possible to determine that the protectively coated substrate 1 can make special results in an improvement of the connection of the adhesive 10 with the protective coating 2 possible during pickling treatment with coordinated parameters with regard to substance amount concentration and pickling duration. 

1. Method for surface treatment of a protectively coated substrate (1), particularly steel strip, for improvement of the adhesion strength, with an adhesive (10), wherein the at least partly multi-phase protective coating (2) comprises at least zinc and magnesium, wherein the protective coating (2) is subjected to a pickling treatment, which is essentially directed at at least one intermetallic Zn—Mg phase (4) of the protective coating (2), with acidic pickle, particularly acid.
 2. Method according to claim 1, wherein in the pickling step for selective pickling of the protective coating (2), an oxide layer (9) of the protective coating (2) is also removed by pickling.
 3. Method according to claim 1, wherein the protective coating (2) is subjected to a pickling treatment essentially directed at MgZn₂.
 4. Method according to claim 1, wherein the protective coating (2), which additionally comprises aluminum, is subjected to a pickling treatment essentially directed at a particularly ternary eutectic (7).
 5. Method according to claim 1, wherein an acid having a pK_(s) value less than 0 is used as an acidic pickle.
 6. Method according to claim 5, wherein a non-oxidizing acid, particularly H₂SO₄, is used as an acidic pickle.
 7. Method according to claim 1, wherein the protective coating (2) is subjected to a pickling duration of 0.5 to 20 seconds, with an acid at a concentration of 1 to 20 volume-%.
 8. Method according to claim 1, wherein the protective coating (2) is subjected to a pickling duration of 1 to 15 seconds, with an acid at a concentration of 2 to 10 volume-%.
 9. Method according to claim 1, wherein the product resulting from the pickling duration in seconds and the concentration in volume-% lies in the range of 1 to
 100. 10. Method according to claim 1, wherein the product resulting from the pickling duration in seconds and the concentration in volume-% lies in the range of 6 to
 75. 11. Use of an acid having a pk_(s) value less than 0 for surface treatment of an object corrosion-protected with a multi-phase protective coating (2) comprising zinc and magnesium, particularly a steel strip (1), for improvement of the adhesion strength, with an adhesive (10).
 12. Use according to claim 11, wherein the acid is used for a pickling treatment essentially directed at the protective coating (2), which additionally comprises aluminum, particularly at its ternary eutectic (7).
 13. Object, particularly a steel strip (1), having a multi-phase protective coating (2) comprising zinc and magnesium, wherein the protective coating (2) has a microstructure reduced by the proportion of intermetallic Zn—Mg phase (4), produced by means of a method for surface treatment according to claim
 1. 14. Object according to claim 13, wherein the protective coating (2) has a microstructure reduced by the MgZn₂ component.
 15. Object according to claim 13, wherein the protective coating (2) that additionally comprises aluminum has a microstructure reduced by the proportion of, in particular, ternary eutectic (7). 